1. Field of the Invention
This invention relates to a method of joining columbium to to titanium and titanium based alloys and, more specifically, to a method of performing such joining under relatively low pressure and low temperature conditions.
2. Brief Description of the Prior Art
Columbium (Cb) is a desirable material for use in environments requiring high temperature regimes (i.e., about 2000.degree. to about 2500.degree. F.), such as for use in thermal protection systems. Columbium and titanium are metallurgically compatible with each other and can be diffusion bonded to each other with high temperatures and pressures. Bi-metallic joints can also be made using T.I.G. and electron beam welding techniques. Prior art in regard to brazing columbium to titanium is set forth in a publication by A. I. Chernitsyn et al., "Svar Proiz", 1975, No. 7, pgs. 26 to 28. Chernitsyn electroplated copper onto the titanium. Nickel was then electroplated on top of the copper. The plated titanium was bonded to the columbian at 1895.degree. F. for 45 minutes with a bonding pressure of about 7.5 ksi. Nickel has a high chemical affinity for columbium and this facilitated the dissolving of the columbium oxide film while the copper formed a large amount of liquid phase with the titanium. The diffusion brazing was completed through an isothermal solidification mechanism resulting in good brazed and diffused joints.
Chernitsyn bonded columbium to titanium by utilizing the Ti-Cu-Ni ternary eutectic liquid (1652.degree. F.) as well as the Ti-Ni binary eutectic (1728.degree. F.) as a brazing medium. The bonding, however, was carried out at 1895.degree. F. This temperature is in excess of the beta-transus of Ti-6Al-4V and the use of this brazing temperature would severely reduce the mechanical properties of Ti-6Al-4V. As the ternary eutectic temperature of Ti-Cu-Ni is only 1652.degree. F., it was considered that a columbium-titanium joint could be produced by brazing below the beta-transus and increasing the time from one to two hours, thereby improving the mechanical properties of the materials after completion of the joint.